Watchdog timer, yes or no and why [SOLVED]

[temtronic]

You might consider putting 74123s between the PIC outputs and the laser drivers.
74123s are retriggerable one shots so if they don't get a pulse(input) after xxx ms they turn off. heck, i don't know if they exist anymore but I used thousands of them 20-30 years ago for a similar application( gas valves in my case.)


Jay

[wangine]

I believe the better solution is external hardware in my case like temtronic say

[Ttelmah]

Yes. This is exactly what I was suggesting with:
"An example would be to have the actual drive hardware built so it always turns itself off. It requires a 'kick' (a pulse signal), perhaps every second, and if it does not receive this 'goes off' automatically. If the pulses stop, the dangerous hardware _turns itself off_".

The NE555, can also be used for this. Very available!...

Set up the timings, so that 'worst case' (temperature, tolerances of parts everything etc.), the on time is a safe period. Then kick it at less than the 'worst case" in the other direction.

This way the system will always go off if the pulses stop.

The 74123, is available now in versions like the 74VHC123A.

The MC14538B is my favourite though. Very good timing accuracies. Wire the reset line to the PIC, with a resistor so the chip is always reset till the PIC turns it on. Really wide range of supported timings, and you can trigger it with a positive or -ve pulse (depending on which input you select).

[wangine]

[Ttelmah]

The classic thing the watchdog _can_ be used to catch, is a spike or particle effect, that leads to the code jumping to the wrong location. Both unlikely with well designed hardware, but with proper trap code, the watchdog can then restart.